In a remarkable discovery, Nasa has found seven Earth-sized exoplanets orbiting a single distant star.

The planets in the extrasolar system, travelling around the TRAPPIST-1 star, are all comparable to Earth in their size, mass, and densities, with three having the potential for an extensive amount of water on their surface.

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"It's really a combination of elements that is quite extraordinary here," Amaury Triaud, from the University of Cambridge's Institute of Astronomy, told WIRED. "By choosing to look at this type of star, which a lot of other researchers chose not to do, we basically took a crucial step to knowing if life has emerged elsewhere in the Universe."

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Individually called TRAPPIST 1b, c, d, e, f, g and h, the seven planets are named in order of their distance from the star, which has an eight per cent mass of our Sun, is just 12 per cent of its size and is 40 light years away from Earth.

Relative to Earth, the planet with the smallest radius (1h) is 0.76 times the size of our planet and the largest is 1g at 1.13 Earth's. Planet 1c (0.41) has the largest mass relative to Earth and 1e has a mass 0.62 of Earth.

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The nearest planet to TRAPPIST-1 (1b) is roughly 149,000 kilometres from the star and takes 1.51 Earth days to complete a single orbit. The scientists weren't able to accurately plot planet 1h's orbit or distance from the star but estimate it takes 12.35 Earth days to complete an orbit and is 673,000km from TRAPPIST-1.

Each of the planets is so close to the others that their gravitational pulls have an impact on how long it takes for them to move around the star. It is thought, although not known for certain, that the planets all have rocky surfaces and their surface temperatures are believed to be between 0 and 100 degrees Celsius.

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According to Triaud, the finding of the system will now help put Earth and our own solar system "in a much broader context" and shows that the type of exoplanets discovered are likely to be more common than predicted.

"We tend to think of Earth and the Sun as common and the "normal" way systems develop but the planets around TRAPPIST-1 could actually be the normal stuff and we are the exotic location," the Cambridge academic added.

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Within the research, published in the journal Nature, the authors explain the discovery is a "unique opportunity" to characterise Earth-like planets that orbit smaller and cooler stars than our own.

Despite the scale of the system, the first details about the TRAPPIST-1 star and three of the exoplanets were revealed previously.

NASA/JPL-Caltech

Michaël Gillon of the Institut d’Astrophysique et Géophysique at the University of Liège, who led the most recent work, announced its discovery in May 2016. At the time the researchers said TRAPPIST-1, also known as 2MASS J23062928-0502285, was a "world first discovery" as the atmospheres of the planets were in reach of telescopes for the first time.

However, after studying the system further, Gillon and colleagues discovered their initial observations were wrong. "The signal we thought was one planet was in fact two other planets," Triaud said. "As time went on, we realised we had a system with a pretty incredible number of planets."

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The star and planets were all found with the robotic telescope called the TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST), based in the Chilean mountains. The telescope started operating in 2010 and is controlled by the Belgian university. During its lifespan it has been involved in the detection of small planets, such as Eris.

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The researchers detected the TRAPPIST-1 exoplanets using a method that inspects how the planets move around their star. Transit Photometry detects planets by measuring the dimming of a star as the planet passes between it and Earth.

During the research the authors used the initial data returned by the TRAPPIST telescope and conducted further surveys using the Hubble Telescope and the Spitzer Space Telescope.

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This poster imagines what a trip to TRAPPIST-1e might be like

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More than 500 hours of Spitzer telescope data was gathered during the analysis of the TRAPPIST-1 system and the SPECULOOS telescope that's being developed will continue this. Now they plan to find out more about the new exoplanets and their atmospheres.

Triaud explains that because the star's light passes through the atmosphere of the planets it is possible to study what gases are contained within them. "Studying the atmosphere allows us to find out its structure, its climate, and how much greenhouse gas there is," he explained.

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NASA/JPL-Caltech

By studying the atmosphere it will be possible to see whether gases are created because of the geography of the planet, or if there is life existing on them. The Hubble Telescope has already started observing what is happening in the atmospheres of the two most inner planets and will provide results in the coming years.

"Could any of the planets harbour life? We simply do not know. But one thing is certain: in a few billion years, when the Sun has run out of fuel and the solar system has ceased to exist, TRAPPIST-1 will still be only an infant star," says a news and views article published alongside the paper.